In today's wireless communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for wireless communication. A wireless communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. Wireless devices, also referred to herein as User Equipments, UEs, mobile stations, and/or wireless terminals, are served in the cells by the respective radio base station and are communicating with respective radio base station. The wireless devices transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the wireless devices in downlink (DL) transmissions.
Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to evolve the WCDMA standard towards the fourth generation (4G) of mobile telecommunication networks. In comparisons with third generation (3G) WCDMA, LTE provides increased capacity, much higher data peak rates and significantly improved latency numbers. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in a DL transmission from a radio base station to a wireless device. Single Carrier-Frequency Domain Multiple Access (SC-FDMA) is used in an UL transmission from the wireless device to the radio base station. Services in LTE are supported in the packet switched domain. The SC-FDMA used in the UL is also referred to as Discrete Fourier Transform Spread (DFTS)-OFDM.
The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes, #0-#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
Downlink and uplink transmissions are dynamically scheduled, i.e. in each subframe the radio base station transmits control information about to or from which wireless devices data is transmitted and upon which resource blocks the data is transmitted. The control information for a given wireless device is transmitted using one or multiple Physical Downlink Control Channels (PDCCH). Control information of a PDCCH is transmitted in the control region comprising the first n=1, 2, 3 or 4 OFDM symbols in each subframe where n is the Control Format Indicator (CFI). Typically the control region may comprise many PDCCH carrying control information to multiple wireless devices simultaneously. A downlink system with 3 OFDM symbols allocated for control signaling, for example the PDCCH, is illustrated in FIG. 3 and denoted as control region. The resource elements used for control signaling are indicated with wave-formed lines and resource elements used for reference symbols are indicated with diagonal lines. Frequencies f or subcarriers are defined along a z-axis and symbols are defined along an x-axis.
Physical Downlink Channels and Transmission Modes
In LTE, a number of physical DL channels are supported. A downlink physical channel corresponds to a set of resource elements carrying information originating from higher layers. The following physical DL channels are supported in LTE:                Physical Downlink Shared Channel, PDSCH        Physical Broadcast Channel, PBCH        Physical Multicast Channel, PMCH        Physical Control Format Indicator Channel, PCFICH        Physical Downlink Control Channel, PDCCH        Physical Hybrid ARQ Indicator Channel, PHICH        Enhanced Physical Downlink Control Channel, EPDCCH.        
PDSCH is used mainly for carrying user traffic data and higher layer messages. PDSCH is transmitted in a DL sub-frame outside of the control region as shown in FIG. 3. Both PDCCH and EPDCCH are used to carry Downlink Control Information (DCI), such as, PRB allocation, modulation level and coding scheme (MCS), pre-coder used at the transmitter, etc. PDCCH is transmitted in the first one to four OFDM symbols in a DL sub-frame, i.e. the control region, while EPDCCH is transmitted in the same region as PDSCH.
Different DCI formats are defined in LTE for DL and UL data scheduling. For example, DCI formats 0 and 4 are used for UL data scheduling, while DCI formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 2D are used for DL data scheduling. In DL, which DCI format is used for data scheduling is associated with a DL transmission scheme and/or the type of message to be transmitted. The following transmission schemes are defined in LTE:                Single-antenna port        Transmit diversity (T×D)        Open-loop spatial multiplexing        Close-loop spatial multiplexing        Multi-user MIMO (MU-MIMO)        Dual layer transmission        Up to 8 layer transmission        
PDCCH is always transmitted with either the single-antenna port or Transmit Diversity scheme, while PDSCH can use any one of the transmission schemes. In LTE, a wireless device is configured with a transmission mode (TM), rather than a transmission scheme. There are 10 TMs, i.e. TM1 to TM10, defined so far for PDSCH in LTE. Each TM defines a primary transmission scheme and a backup transmission scheme. The backup transmission scheme is either single antenna port or T×D. The primary transmission scheme for the 10 TMs are:                TM1: single antenna port, port 0        TM2: T×D        TM3: open-loop SM        TM4: close-loop SM        TM5: MU-MIMO        TM6: Close-loop SM with a single transmission layer        TM7: single antenna: port 5        TM8: dual layer transmission or single antenna port: port 7 or 8        TM9: up to 8 layer transmission, port 7-14 or single antenna port: port 7 or 8        TM10: up to 8 layer transmission, port 7-14 or single antenna port: port 7 or 8        
In TM1 to TM6, cell specific reference signal (CRS) is used as the reference signal for channel estimation at the wireless device for demodulation. While in TM7 to TM10, UE specific demodulation reference signal (DMRS) is used as the reference signal for channel estimation and demodulation. Antenna ports 0 to 3 are CRS ports, while ports 7 to 14 are DMRS ports. TM4 is a CRS based single user (SU) multiple input and multiple output (MIMO) scheme, in which multiple data layers for the same wireless device are multiplexed and transmitted on the same PDB. On the other hand, TM9 or TM10 is a DMRS based SU-MIMO scheme. In TM4 pre-coder needs to be signalled to a UE dynamically. Such information is, however, not required in TM9 and TM10.
Spatial Division Multiplexing (SDMA) or MU-MIMO
When two wireless devices are located in different areas of a cell such that they may be separated through different precoding (or beamforming) at the radio base station, i.e. network node, the two wireless devices may be served with the same time-frequency resources (i.e. PRBs) in a sub-frame by using different beams. A beam is defined by a pre-coder. This approach is called multi-user MIMO, MU-MIMO. In CRS based transmission mode, TM5 may be used for MU-MIMO transmission, in which a wireless device is informed about the MU-MIMO operation. The pre-coder used and the transmit power offset are dynamically signalled to the wireless device through DCI format 1D. In DMRS based transmission modes TM9 and TM10, different DMRS ports and/or the same DMRS port with different scrambling codes can be assigned to the wireless devices for MU-MIMO transmission. In this case, MU-MIMO is transparent to wireless device, i.e., a wireless device is not informed about MU-MIMO, i.e. that another wireless device is scheduled in the same PRB.
In LTE downlink, a number of reference signals (RS) are provided for channel estimation and demodulation purpose. There is one reference signal transmitted per antenna port. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed may be inferred from the channel over which another symbol on the same antenna port is conveyed.
Cell Specific Reference Signals
One DL RS type is Cell specific Reference Signals, CRSs. CRSs are transmitted in every sub-frame and over the entire frequency band. Up to four CRS ports are supported. CRSs are transmitted on a grid of Resource Elements, REs, in each PRB and may be used for downlink channel estimation purpose. An example of the CRS RE locations in a PRB is shown in FIG. 4. The frequency locations of the CRS REs are cell dependent and may be shifted for cells with different physical cell IDs. For channel estimation, the channels on the CRS REs are first estimated. The channels on the data REs are then estimated by interpolation or filtering the channels estimated on the CRS REs.
Since CRSs are cell specific, i.e. they are transmitted to all wireless devices in a cell, but for different wireless devices the downlink transmit power and precoding for PDSCH may be different. Therefore, for correct demodulation and channel quality reporting, the power offset, i.e. relative to the CRS transmit power, and the pre-coder used for PDSCH transmission to a wireless device need to be signalled to the wireless device. Currently, the power offset is semi-statically signalled, i.e. by RRC signalling, to a wireless device using a parameter referred to as PA. The ratio of PDSCH energy per RE, EPRE, to CRS EPRE among PDSCH REs in an OFDM symbol not containing CRS is denoted by ρA. For a UE in transmission modes 1-7, the UE may assume that for 16 QAM, 64 QAM, spatial multiplexing with more than one layer or for PDSCH transmissions associated with the MU MIMO transmission scheme, ρA=δpower-offset+PA [dB], where δpower-offset is 0 dB for all PDSCH transmission schemes except multi-user MIMO, in which δpower-offset is dynamically indicated. One exception is PDSCH data transmission using precoding for transmit diversity with 4 cell-specific antenna ports, in which case ρA=δpower-offset+PA+10 log10(2) [dB].
The range of PA is from −6 dB to +3 dB. Pre-coder is dynamically signalled, i.e. by DCI information in the scheduling message, to a wireless device. In TM5, it is possible to dynamically signal an additional power offset δpower-offset of −3 dB between CRS EPRE and PDSCH EPRE, this is used in case the wireless device is MU-MIMO scheduled with another wireless device in case the PDSCH power per wireless device is reduced by 3 dB.
DL Demodulation Reference Signal (DMRS)
DMRS is also used for downlink channel estimation and demodulation for TM8, TM9 and TM10. Unlike CRS, DMRS is wireless device specific, i.e. it is only transmitted when there is DL data transmission to a wireless device and in those PRBs where the PDSCH is transmitted. There are eight DMRS ports (ports 7 to 15) defined in LTE and thus up to eight layers of PDSCH data may be supported. For wireless devices with a single layer transmission, either port 7 or port 8 can be used. The DMRS port used is dynamically indicated in the associated PDCCH or EPDCCH. The DMRS ports are transmitted on certain fixed REs in a PRB. The RE pattern for port 7 and port 8 are shown in FIG. 4. Ports 7 and 8 occupy the same set of REs in a PRB and the two ports are multiplexed by using orthogonal codes. DMRS is pre-coded with the same pre-coder as the data, so when the wireless device has estimated the channel from DMRS it can directly use the channel estimate for PDSCH demodulation. For DMRS ports 7 and 8, they are also transmitted with the same per RE power as the associated PDSCH data, hence the offset is always 0 dB. Therefore, pre-coder and transmit power offset are not needed at a wireless device for channel estimation and demodulation purpose.
Downlink Power Allocation in LTE
In LTE downlink, the network node determines the downlink transmit power for each wireless device. A wireless device is signalled semi-statically, by RRC signalling the parameter PA, which may have a value of [−6, −4.77, −3 dB, −1.77, 0, 1, 2, 3] dB.
PA represents the baseline transmit power ratio between the PDSCH and the CRS. The wireless device may derive actual transmit power ratio between the PDSCH and the CRS from PA for a given transmission mode, the number of transmit antennas at the network node, the modulation level and the number of layers in a PDSCH transmission.
CRS Based Transmission Modes
In case of CRS based transmission modes (TM1 to TM6), the channel estimation is done through CRS. The transmit power ratio may be derived from a semi-statically signalled parameter PA, and an antenna pre-coder W is either predefined (e.g. TM2 and TM3) or signalled to the wireless device dynamically in each sub-frame (e.g. TM4, TM5, TM6).
DMRS Based Transmission Modes
In case of DMRS based transmission modes (TM7, TM8, TM9, TM10) in LTE, channel estimation is based on UE specific DMRS, which is pre-coded using the same pre-coder and the same transmit power as PDSCH data so the ratio is fixed to 0 dB.
Multi-User Superposition Transmission (MUST)
In LTE up to release 12, only orthogonal multiple access, OMA, is used where wireless devices are multiplexed either time, frequency or spatial domain or a combination of the time, frequency and spatial domains. Another possible form of wireless device data multiplexing currently under study in LTE release 13 is called Multi-User Superposition Transmission, MUST. This is described, for example, in the documents: 3GPP TR 36.859, “Study on Downlink Multiuser Superposition Transmission for LTE”; 3GPP R1-152493, Huawei HiSilicon, “Candidate schemes for superposition transmission,” May 2015; 3GPP R1-153333, NTT DOCOMO, “Candidate non-orthogonal multiplexing access scheme,” May 2015; 3GPP R1-151425, Qualcomm Incorporated, “Multiuser superposition schemes,” April 2015; and 3GPP R1-153332, NTT DOCOMO, “Evaluation methodologies for downlink multiuser superposition transmissions,” May 2015.
In MUST, two (or more) wireless device with different path losses, or SINR, to a network node, e.g. a serving radio base station or eNB transmitter, are superposed on the same time-frequency and/or spatial resources. This may be realized by assigning different transmit powers to different wireless devices. The transmit power level allocated to a given wireless device is generally determined by the channel condition (i.e., path loss) experienced by the wireless devices. For instance, wireless devices having higher path loss, e.g. wireless devices located far away from the network node, may be allocated higher transmit powers, while wireless devices having lower path loss, e,g, wireless devices located close to the network node, may be allocated lower transmit powers. The total combined transmit power may, however, be kept the same.
One example is shown FIG. 5. In FIG. 5, a first wireless device, UE1, located a first distance from the network node and a second wireless device, UE2, located a second distance from the network node are present in a cell of a wireless communications network. Since the first wireless device, UE1, is located closer to the network node than the second wireless device, UE2, i.e. the first distance is shorter than the second distance, the first wireless device, UE1, may also be referred to as a near wireless device or near, while the second wireless device, UE2, may also be referred to as a far wireless device or far UE. However, it should be noted that this illustration is just one example. Generally, a “near UE” is not necessarily physically closer to the network node than the “far UE”. For example, a “far UE” may be inside a building and has poorer received signal than a “near UE” which has a line of sight path to the network node. In this case, the “far UE” may be physically closer to the network node than the “near UE”. So, the terms “near UE” and “far UE” are herein used to indicate the relative signal quality received at a UE, that is, a “near UE” has a better received signal quality than a “far UE”.
The two wireless devices, UE1 and UE2, may, for example, be superposed at the same time-frequency resource according to Eq. 1:x=√{square root over (P1)}s1+√{square root over (P2)}s2  (Eq. 1)    where x is the superposed signal transmitted from the network node, and            Pi is the allocated transmit power to the wireless devices, UE i (i=1,2). Also,        
                    ∑        i            ⁢                          ⁢              P        i              =    P    ,where P is the total transmit power over the resource element.
The received signal y at the wireless devices, UE i (i=1,2) may then be described according to Eq. 2:yi=Hi·(√{square root over (P1)}s1+√{square root over (P2)}s2)+vi  (Eq. 2),or according to Eq. 3:yi=Hi·√{square root over (P)}(√{square root over (α1)}s1+√{square root over (α2)}s2)+vi  (Eq. 3)    where Hi (i=1,2) is the channel response to UE i, and
            α      1        =                                        P            1                    P                ⁢                                  ⁢        and        ⁢                                  ⁢                  α          2                    =                        P          2                P              ,            v      i        ⁡          (                        i          =          1                ,        2            )      is the receiver noise at the wireless devices, UE i (i=1,2).
FIG. 6 shows the received signal power at each of the wireless devices, UE i (i=1,2). Since the first wireless device, UE1, is closer to the network node i.e. a cell centre wireless device, than the second wireless device, UE2, being far away from the network node i.e. a cell edge wireless device, the first wireless device, UE1, will have a smaller propagation path loss compared to the second wireless device, UE2, which will have a larger propagation path loss. To reach the second wireless device, UE2, a higher transmit power is needed than for the first wireless device, UE1, i.e. P2>P1. By P1 being much smaller than P2, the second wireless device, UE2, may still be able to decode its data successfully at the presence of signal of the first wireless device, UE1.
Since the first wireless device, UE1, is close to the network node the first wireless device, UE1, will experience a strong signal intended to the second wireless device, UE2. If the first wireless device, UE1, may estimate the signal H1·√{square root over (P2)}s2, then the first wireless device, UE1, may cancel this estimate from the received signal y1. After the cancellation, the first wireless device, UE1, would be able to decode its own signal.
A general MUST transmitter and receiver are shown in FIGS. 7-8, respectively. When the network node has multiple transmit antennas, each of the signals may be pre-coded before transmission. In this case, the transmitted signal from the network node becomes x according to Eq. 4:x=√{square root over (P1)}W1s1+√{square root over (P2)}W2s2  (Eq. 4)    where x=[x1, x2, . . . , xNTX]T and xn(n=1, . . . , NTX) is the transmitted signal on the nth antenna,            NTX is the number of transmit antennas,        Wi (i=1,2) is a NTX×1 precoding vector applied to the signal si.        
If the first and second wireless device, UE1 and UE2, also have multiple receive antennas, the received signal at UE i (i=1,2) becomes y according to Eq. 5:yi=Hi·x+vi=Hi·(√{square root over (P)}1W1s1+√{square root over (P2)}W2s2)+vi  (Eq. 5)    where yi=[yi(1), yi(2), . . . , yi(NiRX)]T, yi(k) is the received signal on antenna k of UE i,            NiRX is the number of receive antennas of UE i;        Hi is a NiRX×NTX channel matrix, and        vi is a NiRX×1 noise vector.        
Similar to the single antenna case, if the first wireless device, UE1, may, by using the channel estimate Ĥ1 and information about √{square root over (P2)}W2, estimate the transmitted signal √{square root over (P2)}W2s2, then the first wireless device, UE1, is able to decode its own signal after subtracting Ĥ1·√{square root over (P2)}W2s2 from the received signal y1=H1·(√{square root over (P1)}W1s1+√{square root over (P2)}W2s2)+v1. Ĥ·√{square root over (P1)}W1 may be referred to herein as the estimated effective channel associated with UE1 observed at UE1, and Ĥ·√{square root over (P2)}W2 may be referred to herein as the estimated effective channel associated with UE2 observed at UE1. Similarly, Ĥ2·√{square root over (P2)}W2 may be referred to herein as the estimated effective channel associated with UE2 observed at UE2, where Ĥ2 is the estimated channel at UE2.
MUST Transmission Schemes
Three variants of MUST schemes are being considered in the LTE Release 13 study item on MUST, see for example, 3GPP TR 36.859, “Study on Downlink Multiuser Superposition Transmission for LTE”. Brief descriptions of these schemes are given below.
Non-Orthogonal Multiple Access (NOMA)
In the NOMA scheme, the information bits corresponding to the far UE, i.e. the second wireless device, UE2, and the near UE, i.e. the first wireless device, UE1, are independently encoded and modulated. The symbol s1 is drawn from a near UE constellation and the symbol s2 is drawn from a far UE constellation. Then, the superposed symbol x in the NOMA scheme has a superposed constellation (e.g. a super-constellation).
One example of the superposed NOMA constellation for the case where both the near UE, i.e. the first wireless device, UE1, and far UE, i.e. the second wireless device, UE2, employ QPSK constellation is shown in FIG. 9. In this case, the superposed constellation is similar to a 16QAM constellation.
Semi-Orthogonal Multiple Access (SOMA)
The SOMA scheme differs from the NOMA scheme in that the SOMA scheme uses Gray mapped superposed constellation. The coded modulation symbols of near UE and far UE, i.e. of the first and second wireless device, UE1 and UE2, are jointly Gray mapped and then added together, such as, e.g. in Eq. 1 shown above.
One example of the superposed SOMA constellation for the case where both the near UE, i.e. the first wireless device, UE1, and far UE, i.e. the second wireless device, UE2, employ QPSK constellation is shown in FIG. 10. In this case, α=α1.
Rate-Adaptive Constellation Expansion Multiple Access (REMA)
The REMA scheme is similar to the SOMA scheme, however, with one restriction that the resulting superposed constellation should be a regular QAM constellation having equal horizontal and vertical spacing between constellation points (as is used in e.g. LTE).
In the REMA scheme, the bits with the higher bit-level capacities are allocated for the far UE, i.e. the second wireless device, UE2, and the bits with the lower bit-level capacities are allocated for the near UE, i.e. the first wireless device, UE1. In addition, a power sharing parameter may also be set appropriately so that the resulting superposed constellation is a regular QAM constellation.
There are six different ways (as shown in Table 1 below) of realizing the REMA scheme that has LTE standard constellations as superposed constellations. FIG. 11 shows one example of a 16-QAM superposed REMA constellation.
TABLE 1NearFarUE PowerUE PowerSuperposedFar UENear UEShare α1Share α2ConstellationConstellationConstellationin dBin dB16-QAMQPSKQPSK −6.9867 dB−0.9691 dB64-QAMQPSK16-QAM −6.2342 dB−1.1805 dB64-QAM16-QAMQPSK−13.1876 dB−0.2136 dB256-QAMQPSK64-QAM −6.0730 dB−1.2321 dB256-QAM16-QAM16-QAM−12.2915 dB−0.2641 dB256-QAM64-QAMQPSK−19.2082 dB−0.0524 dBNetwork Node Scheduling
In each sub-frame and each scheduling sub-band, the network node may schedule wireless devices using either OMA transmission or MUST transmission depending on whether or not a suitable wireless device pair can be found for a MUST scheduling based on some scheduling metric, such as, e.g. Proportional Fairness, PF.
If there is a suitable pair of wireless devices found in a sub-frame, then MUST transmission may be scheduled. Otherwise, an OMA transmission may be scheduled.
One example is shown in FIG. 12, wherein a suitable wireless device pair, UE1 and UE2, are found and a MUST transmission is scheduled for these two wireless devices in the sub-frame (k+2). In the remaining sub-frames, OMA transmission is scheduled.